1. Field of the Invention
The present invention relates to a signal transmission circuit cable, and in particular to a differential mode signal transmission module for transmitting differential mode signals.
2. The Related Arts
General processes for manufacturing flexible circuit boards can be classified as (1) a flexible board formed by transferring image to and etching a conductor layer and laminating an insulation layer, (2) a flexible board formed by directly printing a conductor layer on an insulation substrate, and (3) a standard flexible circuit cable formed by stretching a plurality of straight and parallel conductor wires by a coiling machine and laminating an insulation layer. Classification made according to functionality may include carrier boards, flat cables, and flexible printed circuit board featuring functions of both carrier board and flat cable. The flexible circuit boards bear numerous different names, such as flexible printed circuit boards and flexible flat cables, and they all refer to the same product.
A flexible circuit cable easily generates electromagnetic waves of high frequency and high energy in transmitting signals. Thus, the signal transmitted is also susceptible to interference by external high frequency noises and signal distortion may result. Further, a flexible flat cable or a circuit board often generates high voltage electrostatic discharge due to extension through a bore in movement and fast and effective grounding connection is of vital importance in this situation.
To cope with such a problem, a known solution is to cover a metal shielding layer on a surface of a flexible circuit board to block the external electromagnetic noises. Although such a metal shielding layer is effective in suppress the interference caused by the external electromagnetic noises, yet the covering metal shielding layer is not actually connected the ground of an electronic device, so that the effect of the metal shielding layer for shielding electromagnetic noises and eliminating electrostatic discharge is generally poor.
Thus, the existing electronic devices commonly use the conventional flexible circuit boards that are made through etching processes and are easily connectable to ground for signal connection. However, the conventional flexible circuit boards are of costs that are much higher than the flexible circuit cables. Further, the reason that the flexible circuit cables do not provide an effect of electromagnetic noise shielding is that the manufacture thereof is done by stretching conductor wires followed by laminating, and thus, the conductor wires cannot be made thin, which makes control of impedance difficult. Thus, it is now an immediate challenge for the industry to reduce cost by using flexible circuit cable to partly replace the use area of the conventional flexible circuit board and to also provide a conduction structure that eliminates electrostatic discharge. In addition, applications of some of these products require extension through a narrow hole or a hinge bore and bending resistance test for being bent tens of thousands times. Partial slitting to form a bundled structure is also inevitable.
Further, in the various modern electronic devices, the amount of data transmitted through signal lines is increasingly expanded. Thus, the number of signal transmission lines used is increased and the frequency of the transmission signal is also getting higher. Consequently, a differential mode is often used in high frequency transmission to reduce electromagnetic interference (EMI). For example, USB or LVDS signals are commonly transmitted with such a technique to reduce EMI. Referring to FIG. 1, a schematic view is given to illustrate a conventional flexible circuit cable transmitting a differential mode signal. The differential mode signal transmitted by a flexible circuit cable A includes a plurality of differential mode signal transmission terminals S. Each of the differential mode signal transmission terminals S is composed of a first differential mode signal conductor wire Sa, a second differential mode signal conductor wire Sb, and a grounding conductor wire Ga. The first differential mode signal conductor wire Sa and the second differential mode signal conductor wire Sb form two-paired wires, called a differential pair, capable of transmitting two complementary signals that have identical levels but opposite phases. The technique of differential mode signal transmission improves resistance to electromagnetic noise. Since two conductor wires are placed very close and the amplitudes of the signals are identical and since the amplitudes of the coupling electromagnetic fields with respect to grounding lines are also identical, with the phases of the two conductor wires being opposite, the electromagnetic fields thereof will cancel each other and thus the influence thereof by external electromagnetic interference is reduced.
However, when a flexible circuit cable is used to transmit a differential mode signal, the number of the conductor wires thereof is the summed number of the plurality of differential mode signal transmission terminals S, plus at least one power line P and a primary grounding line Gp. If the differential mode signal transmission terminals S are of a relatively large number, the total number of the conductor wires of the flexible circuit cable is increased and will occupy a large amount of space of a circuit in a practical application. If the flexible circuit cable must be set through a hinge or a narrow hole, the excessive number of the conductor wires makes it difficult to smoothly extend through the bore of a hinge member or a narrow hole. Further, the increase of the number of the conductor wires may lead to shortcomings, such as increased manufacturing cost, increased weight, and expansion of wire volume of the electronic device.